Generally speaking, linear positioning is useful in a number of technical, industrial and commercial applications. For example, linear positioning of lenses provides zoom focus capability in scanners, which may be used for reading bar code patterns and documents, from which data presented in two dimensional (2D) graphic media may be accessed.
Linear position control is provided by systems that have actuation components (“linear actuators”). The simplest linear actuators move a load item (“load”) from a first position to a second position along an axis of translational motion. The load may also be moved from the second position to the first position.
Some linear actuators may thus be operable in a forward direction (from the first position to the second position) and a reverse direction (from the second position to the first position). Somewhat more complex linear actuator may move the load, forward and reverse, between the first and the second positions, and to one or more additional positions.
For example, linear actuators used with three (3) position mechanical zoom move the load to a third position. The third position may be disposed along the translational axis at an intermediate point between the first position and the second position, which thus correspond to opposite extremes of movement of the load.
The actuator may move the load back and forth between hard stops at each of the first and the second positions, but not beyond either. Further, the actuator may move the load to a hard stop at the intermediate third position, from which it may then move the load to either of the first or the second positions.
The hard stops comprise positions at which the movement of the load is stopped and temporarily constrained from moving further in either direction. The hard stops of the first and the second positions are disposed at opposite fixed positions along the axis, which the linear actuator is constrained not to exceed.
The hard stop of the third position may correspond to a particular intermediate location as precise as the fixed locations of the first and the second positions. However, the actuator is operable for moving the load from the stop at the third intermediate position to the first and/or to the second position.
Lacking the fixed locations of the first and the second positions, the location of the intermediate third position must be designated with a precision level sufficient for a given use application. Effectively reliable linear positioning demands consistent repeatability in the achieving the sufficient precision level.
To attain the consistent repeatability, one or more additional position indicating components (“position sensors”) may typically be used with the linear positioning system. The position sensors are operable for designating the precise location along the linear translational axis at which to stop the load in the intermediate position.
As the actuator moves the load over the linear axis, the position sensor tracks the load's changing position. Upon sensing that the load has been moved into the precise intermediate position, the motion of the load may be selectively stopped, and held in that position for as long a duration as may be selected.
Stopping the load in the first position and the second position is relatively simple, as these opposite motion limits are fixed. The complexity level rises significantly however in relation to stopping the load at the comparatively non-fixed intermediate third position. For example, the sensor first tracks the load as it is moved by the actuator.
Responsive to detecting that the load reaches the intermediate position, the sensor functions to trigger a stoppage of the motion of the load in that position. However, effects such as latency related to combining the operations of the position sensor and triggering the stop may impact the achievable precision level and/or its repeatability.
Moreover, the addition of the position sensors adds cost and complexity to the linear positioning systems. In addition to the impacted precision or reliability, the increased complexity of the linear positioning systems may add concomitant reliability issues or exacerbate existing ones associated therewith.
The additional cost and complexity associated with adding the position sensors to the linear positioning systems may be prohibitive for use in some applications. For example, 3-position mechanical zoom features may add significant functionality to simple, inexpensive optical scan engines, if sufficient precision is achievable consistently.
The linear position control in these scan engines must function at a designated level of precision and repeatability in delineating the intermediate position for stopping the movement of the load by the linear actuators. However, adding position sensors to the linear positioning systems of such scanners raises their cost and complexity prohibitively.
Moreover, latency and other precision and repeatability related effects added by the use of the position sensors with the linear positioning systems may also complicate their design and construction. The added complication may, of course, raise costs further and pose concomitant additional reliability issues.